Such a type of communications network system conventionally known is based on an STM (synchronous transfer mode).
FIG. 10 is a block diagram showing a conventional communications network based on STM (synchronous transfer mode). The communications network system of FIG. 10 has four node devices 1-1 to 1-4 connected through optical transmission lines 1a, 1b and the node device 1-1 determined as a master station.
The node device 1-1 as the master station is connected to an operation management device 2-1 which collects information on operation conditions of the node devices 1-1 to 1-4 and the optical transmission lines 1a, 1b and displays the collected results on, for example, a screen.
The node device 1-1 as the master station in the configuration shown in FIG. 10 is shown in detail in the block diagram of FIG. 11.
In FIG. 11, the node device 1-1 as the master station comprises an optical transmission line interface 3 connected to the optical transmission lines 1a, 1b, a frame generation part 4 which generates a transmission frame and circulates it through the respective node devices, a frame termination part 5 which examines a transmission frame having come back through the respective node devices, a control part 6 which is connected to the operation management device 2-1 to control comprehensively the entire operation of the node device 1-1, and an interface part 7 connected to an external interface.
In the above-described communications network system based on STM (synchronous transfer mode), the respective node devices 1-1 to 1-4 must synchronize at intervals of, for example, 125 .mu.S; then as shown in FIG. 11, the node device 1-1 as the master station needs the frame generation part 4 to generate a transmission frame at intervals of 125 .mu.S and circulate it through the respective node devices. Besides, the node device 1-1 as the master station needs the frame termination part 5 in order to examine the transmission frame having come back through the respective node devices and to transmit it again at intervals of 125 .mu.S.
To improve reliability of the system, it is necessary to have a substitute master station, for example, the node device 1-3 shown in FIG. 10, which can take the place of the node device 1-1 as the master station in case that the node device 1-1 as the master station has a trouble. And, the node device 1-3 to be a substitute master station also needs to have the frame generation part and the frame termination part. In this case, the node device 1-3 to be the substitute master station is connected to an operation management device 2-2 same as the operation management device 2-1.
In the configurations shown in FIG. 10 and FIG. 11, if there were a trouble in the optical transmission lines 1a, 1b or the optical transmission line interfaces of the respective node devices 1-1 to 1-4, it is detected by a neighboring node device, and information on the trouble is put on the transmission frame, which is circulating through the respective node devices, to inform the node device 1-3 to be the substitute master station of the trouble.
The node device 1-3 to be the substitute master station having received the information on the trouble puts information for instructing to change the transmission lines or to loop back on the transmission frame circulating through the respective node devices, and the respective node devices having received the information perform processing according to the information.
Thus, if a conventional communications network system based on STM (synchronous transfer mode) had a trouble, the node device 1-3 to be a new master station operates to reconfigure the network having the trouble, but there is a disadvantage that it takes a long time from the occurrence of a trouble to the rebuilding of the system.
And, if the node device 1-3 to be the master station had a trouble, it cannot generate a transmission frame or mark its own number in the transmission frame whenever the transmission frame is generated. In such a case, a plurality of other substitute master stations detect the situation, mark their own numbers in the transmission frame to generate respective transmission frames, and circulate such transmission frames. Upon receiving such transmission frames, the other substitute master stations, if their own numbers (priority) are larger than those of the other substitute master stations, abandon the received transmission frames but if smaller than them, pass on them as they are. And, when the substitute master station receives the returned transmission frame having its own number marked, this substitute master station becomes a new master station.
As described above, the conventional communications network system based on STM (synchronous transfer mode) circulates the transmission frames to decide a master station. Therefore, it takes a long time to decide a new master station after the occurrence of a trouble in the master station, resulting in a drawback of taking a long time to reconfigure the system.